The invention relates to photolithography instruments used for patterning and processing substrates such as semiconductor chips and wafers. More specifically, the invention is concerned with reducing distortion of the reticle and the corresponding reticle pattern.
Lithography processes require positioning a reticle between an energy beam (typically electron or light) and the substrate chip, or wafer. The reticle must be held without slippage and in a way that does not cause distortion of the reticle pattern. This reticle is typically very thin. This thinness can cause the reticle to deform rather easily even though the reticle usually includes material reinforcing its perimeter, sometimes in the form of a ring secured to its perimeter. If a reticle deforms it can produce an imperfect image on the substrate that results in an imperfect final product.
In modern lithography processes for exposing patterns on wafers and other substrates the reticle is moved at high speeds between discrete and precise positions to facilitate focusing the image on the substrate. This motion can generate dynamic reaction forces where the reticle is supported, leading to distortion of the reticle and, hence, distortion of the image focused on the substrate. It is therefore critical that the reticle be chucked in a manner that reduces or eliminates such reaction forces. The problem is complicated by the fact that lithography processes may occur in a clean room vacuum environment, rendering pneumatic chucks ineffective.
Various attempts to address this difficulty have been less than successful. For example, reaction forces generated by the wafer (substrate) stage motion can be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,528,118 and published Japanese Patent Application Disclosure No. 8-166475. Additionally, reaction forces generated by the reticle (mask) stage motion can be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,874,820 and published Japanese Patent Application Disclosure No. 8-330224. These proposed solutions, however, have not provided optimum results.
As the preceding discussion implies, a lithography process is a complex interaction of sensitive subsystems. The reticle and chuck assembly subsystem is sensitive to numerous inputs. Many things can contribute to the distortion of the reticle such as motion or vibration of the support structure for the reticle, or heat from an electron beam, which may cause thermal expansion. Prior art chuck assemblies can impart stresses where they clamp the reticle. In addition, such chuck assemblies that hold the reticle rigidly in all dimensions can compound deformation caused by thermal expansion by forcing the reticle to twist or bow as it expands.
Other known methods of holding the reticle are often also not satisfactory solutions. Electrostatic methods of holding the reticle such as described in U.S. Pat. No. 5,532,903 to Kendall can be less secure than necessary considering the accelerations of 40 m/sec2 or more that the reticle undergoes during the lithographic process. Vacuum methods of holding the reticle are ineffective in processes because typically the process itself is conducted in a vacuum.
Thus, there is a continuing need in the art for a chucking assembly and method for the reticle retention that eliminates the stresses involved with mechanical clamping and allows for thermal expansion, while simultaneously holding the reticle with enough force that the reticle accelerations associated with the process do not degrade the final product.
The present invention provides a chuck assembly for connecting a reticle to a stage while limiting distortion of the reticle and while still allowing movement with the stage. The chuck assembly includes a number of connector members secured to the stage where each member permits a seat to translate along a single axis in a plane parallel to the movement of the stage. The chuck assembly also includes protrusions that are secured to the reticle and that are also received by the seats on connector members. Using the protrusion and seat arrangement removes or reduces clamping forces as a source of reticle distortion. The interface between protrusion and seat also allows for some degree of misalignment while still providing the degree of retention and control necessary to position the reticle.